Hitherto, image forming apparatuses, such as electrophotographic printers, expose a photosensitive member to a laser beam that is controlled in accordance with an image signal, thereby forming an electrostatic latent image corresponding to the image signal on the photosensitive member. The image forming apparatuses then develop and transfer the formed electrostatic latent image on recording media, such as paper, thereby forming an output image. Such image forming apparatuses have a problem that shifting of a print position is caused by various factors and significantly damages the quality of images.
For example, when unevenness occurs in rotation speed of an image bearing member, such as a photosensitive member, a position in a sub scanning direction, in which the image bearing member is scanned with a laser beam or the like, shifts and unevenness is caused in intervals between sub scanning lines. Because of this, horizontal bands (hereinafter, referred to as banding) resulting from a gradation of density are caused in an output image. More specifically, even if uniform exposure is attempted over the whole surface of the photosensitive member, an amount of exposure in a predetermined area decreases and the density decreases because the scan-line interval widens at high rotation speed. Additionally, since the scan-line interval narrows at low rotation speed, the amount of exposure in the predetermined area increases and the density increases.
As a first method for solving such a problem, a method for correcting density of input image data in consideration of density unevenness information that is previously detected (e.g., Japanese Patent Laid-Open No. 11-112810) has been proposed. In addition, as a second method, a method for correcting a position of image data in consideration of a print-position shifting characteristic (e.g., U.S. Pat. No. 6,625,331) has been proposed.